1. Field of the Invention
The present invention relates in general to the field of marine seismic surveying and in particular to the process of marine seismic surveying utilizing two vertically spaced streamers. Still more particularly, the present invention relates to method for enhancing the accuracy of seismic surveying by providing a method for eliminating secondary reflections of seismic waves downward from the surface of the body of water in which the marine survey is being taken.
2. Description of the Prior Art
Seismic surveying is well known in the prior art. Such surveying typically involves the utilization of a source of seismic energy and one or more arrays of seismic detectors which are strategically positioned to receive the seismic waves generated by the source of seismic energy. The source of seismic energy typically utilized may be an apparatus capable of delivering a series of impacts or mechanical vibrations to the surface of the earth or the detonation of a high explosive charge near the earth's surface. The resultant acoustic waves which are generated in the earth, including those which are reflected from the various strata interfaces within the formations of the earth, are then detected by seismic detectors which convert these acoustic waves into representative electrical signals. From these electrical signals, data may be deduced concerning the structure of these strata within the earth's surface.
Marine seismic surveying operates in much the same manner. An explosive device or vibration inducing compressed air system is typically utilized to generate seismic energy which then propagates as seismic waves into the earth formations below the surface of a body of water. Reflections of this seismic energy from the various strata within the earth are then detected at a plurality of seismic receivers which are generally coupled together in so-called "streamers" and towed behind a marine craft. It is well known to utilize more than one streamer, each of which may contain over 10,000 seismic detectors. Further, multiple streamers are often towed in a vertically spaced alignment to permit variations due to the proximity of the water's surface to be factored out.
A problem in all forms of marine seismic processing occurs due to the fact that seismic wave energy, after reflecting from subterranean strata and traveling upward to the detectors within a streamer, will often reflect again at the water/air interface and travel downward, once again passing the detectors within a streamer. The complex nature of seismic wave signals makes interpretation difficult under the best of conditions; however, the addition of a downward traveling seismic wave renders this process exceedingly difficult.
A similar problem has occurred in so-called vertical seismic profiling or "VSP" wherein multiple seismic detectors are suspended beneath the surface of the earth in a borehole and seismic energy is imparted to the earth at the surface. As described above, with this situation the seismic waves which impact the detectors suspended within the borehole may be traveling upward due to reflections from subterranean strata or downward as a direct seismic wave generated by the seismic energy source.
U.S. Pat. No. 4,794,573, issued to Bell et al., and assigned to Conoco, Inc. of Ponca City, Okla., proposed a solution to this problem in VSP applications which is based upon the concept that waves traveling in opposite directions have spatial derivatives of opposite sign. In accordance with the Bell et al. method the derivative of the seismic signal is approximated by the difference of two signals from vertically spaced detectors which is then time integrated to recover the phase. The resulting integrated difference signal is then amplitude scale corrected and combined, by addition or subtraction, with a signal which represents the sum of the two detector signals to form a succession of filtered signals, which, when recorded in alignment in order of detector depths, form a vertical seismic profile which may preserve either the upgoing or downgoing seismic events.
The Bell et al. process represents a substantial improvement in the vertical seismic profiling environment; however, the accuracy of the process has been found to suffer due to the sensitivity of the integration process to the presence of small direct current bias within the recorded traces or low frequency components in general. Further, during vertical seismic profiling the detectors are spaced a fixed distance apart. This distance is known and may utilized to scale the integrated difference signal. In contrast, in a marine seismic environment the distance between two vertically displaced detectors within two seismic streamers will vary due to the dynamic nature of the process involved in towing a seismic streamer through a body of water. Thus, merely scaling the integrated difference signal as proposed in the process cannot be easily accomplished without introducing additional errors into the data.
It should therefore be apparent that a need exists for a process whereby upgoing and downgoing seismic waves may be easily separated during marine seismic surveying whereby the downward traveling reflections of seismic waves from the water/air interface will not adversely affect the accuracy of the recorded seismic events.